A simple method of evaluating margin-growing accuracy in image-guided radiation therapy.

Objective: In order to not add extra uncertainty to radiotherapy planning process, accurate margin algorithm is necessary, therefore we propose a centre-shift method to estimate its accuracy. Methods: A series of spherical phantoms are used with different CT slice separations (SSs) and pixel sizes (PSs). They are grown by different margins, and displaced geometric centres provide a window on accuracy. Volume difference between pre- and post-expansion is also calculated to double-check the accuracy. Results: The measured margin nearly varies as multiples of PS in the transaxial plane; in the superior-inferior direction, it is approximately equal to SS when the ideal margin is smaller than SS. A sphere's volume of <1cm3 underestimated by 3-70% for all PSs, and 2-100% for SS of ≤5mm but overestimated up to 112% for >5mm. For volumes of >1 cm3, relative volume error decreases, and it is nearly zero for >100cms. The dependence of margin accuracy on SS and PS is largely eliminated by volume difference method. Conclusion: We have proposed a simple method to estimate margin-growing accuracy and suggested corrective action to minimize the variation. Advances in knowledge: One big difference from the previous results is that SS and PS both influence the accuracy of margin growth and volume calculation in Eclipse™ treatment planning system (Varian Medical System. Palo Alto. CA). [ABSTRACT FROM AUTHOR]